This invention relates to nitride based blue laser diodes.
Nitride based blue laser diodes are being developed for printing and optical data storage applications. The first AlGalnN blue laser diodes were broad area lasers providing no control over the laser diode""s various spatial modes. Most applications, however, require the laser diode to operate in a single spatial mode. One way of achieving single spatial mode operation for AlGalN blue laser diodes is to use a ridge waveguide structure to define a lateral waveguide as described in xe2x80x9cRidge-geometry lNGaN multi-quantum-well-structure laser diodesxe2x80x9d by S. Nakamura et al., in Applied Physics Letters 69 (10), pp. 1477-1479 which is hereby incorporated by reference in its entirety. While a ridge waveguide provides for single spatial mode emission in blue lasers, the waveguiding provided is relatively weak. The lateral refractive index step is small and is influenced by heating and carrier injection. Additionally, there are fabrication difficulties because the ridge must be etched to extend sufficiently close to the laser active region without the ability to use an etch stop to prevent material damage to the laser active region since chemical etching is not applicable to GaN materials.
To provide stronger mode stability and low threshold current operation, more strongly index-guided diode lasers are required such as those having buried heterostructures that are typically used for lNGaAsP fiber optic-communication lasers, or the impurity-induced-layer-disordered waveguide structures used for high-power single-mode AlGaAs laser diodes. Additionally, the use of a buried heterostructure avoids certain fabrication difficulties.
Both index-guided buried heterostructure AlGalnN laser diodes and self-aligned index guided buried heterostructure AlGalnN laser diodes provide improved mode stability and low threshold current when compared to conventional ridge waveguide structures. A structure for the index-guided buried heterostructure AlGalnN laser diode in accordance with the invention typically uses insulating AlN, AlGaN or p-doped AlGaN:Mg for lateral confinement and has a narrow (typically about 1-5 xcexcm in width) ridge which is the location of the narrow active stripe of the laser diode which is defined atop the ridge. The narrow ridge is surrounded by an epitaxially deposited film having a window on top of the ridge for the p-electrode contact. The ridge is etched completely through the active region of the laser diode structure to the short period superlattice n-cladding layer. The short period superlattice is used to allow adequate cladding layer thickness for confinement without cracking. Typically, use of a short period superlattice allows doubling of the cladding layer thickness without cracking. This reduces the intensity of the light lost due to leakage by about 2 orders of magnitude with an accompanying improvement in the far-field radiation pattern in comparison with conventional structures. Junction surfaces are exposed by the ridge etch and these junction surfaces contribute surface states which prevent injected carriers from filling conduction or valence band states needed for a population inversion. However, the epitaxial regrowth of a high bandgap material passivates the surface states because the interface between the overgrown material and the ridge structure is perfectly coherent.
The structure for the self-aligned, index guided, buried heterostructure AlGalnN laser diode uses the p-cladding layer to also function as the burying layer to provide strong lateral optical confinement and strong lateral carrier confinement. The p-cladding layer/burying layer is typically AlGaN:Mg. The structure for the self-aligned, index guided, buried heterostructure laser diode is simpler than for the index-guided, buried heterostructure AlGalnN laser diode. The laser structure is grown through the active quantum well and waveguide region followed by etching a narrow laser ridge down to the n-bulk cladding layer. The p-type cladding/burying layer is then overgrown around the ridge along with the p-contact layer. Subsequent laser processing is simple since the two-step growth process results in a lateral waveguide and carrier confinement structure which does not require the creation of contact windows. Hence, the laser processing required is basically a broad area laser fabrication sequence. Additionally, the comparatively large p-contact area allowed by the self-aligned architecture contributes to a lower diode voltage and less heat during continuous wave operation of the laser diode.